In information storage and retrieval systems, the following characteristics are desirable: (1) to minimise the size of a distinguishable feature in the record (in digital records this is a "bit"); (2) to minimise the energy requirement to write one bit; (3) to maximise the writing speed or number of bits written per second; (4) to maximise the reading speed of recorded information; (5) to maximise the signal level obtained when reading recorded information; (6) to maximise signal-to-noise ratio during reading; and (7) to maximise the contrast between readout of `1` and `0` bits.
Several magneto-optic techniques are in theory suitable for use in information storage. These include magnetic wiring (i.e. recording) followed by optical reading of the recorded material; thermomagnetic writing/magnetic reading; and thermomagnetic writing/optical reading. The present invention is directed particularly to systems using optical reading or thermomagnetic writing (or both). Where the writing step is a thermomagnetic step, the physical effect produced in the medium by writing (i.e. by the recording process) is, at least in part, magnetic and may for example involve local heating of the medium by the absorption of incident electromagnetic radiation. It is one object of the present invention to provide a magneto-optic memory structure with high sensitivity to writing radiation so that the energy requirement for writing one bit is reduced and the writing speed increased.
Where the readout of magneto-optic memory structures is optical, it may be achieved by observing the change in polarisation state of electromagnetic radiation scattered from the surface. The scattering may be direct transmission or reflection, or scattering in other directions. When the readout radiation is linearly polarised, the change in polarisation state of the scattered light may usually closely be described as a rotation of the polarisation direction. This is closely equivalent to the addition of a polarisation component orthogonal to the incident polarisation in the scatter beam (the added component being the so-called "magneto-optic" component).
This magneto-optic component with orthogonal polarisation "carries" the readout information, but is generally of very small intensity. The scattered component with polarisation parallel to that of the incident radiation which carries no information is however, very large. These factors lead to low readout signal-to-noise ratio and low readout contrast which thus limits the usefulness of known magneto-optic memories by reducing the ability to extract stored information from them. In digital storage where "bits" are stored, this can result in a "Bit Error Rate" which is larger than is desired for many applications. It is a second object of the present invention to provide means by which the readout signal-to-noise ratio and contrast may be improved in order to improve the readout "Bit Error Rate".
It is a third object of the present invention to realise the improvements in recording sensitivity and readout "Bit Error Rate" using a magnetic-optic structure that may be formed by reliable production techniques.
There are known in the art several multilayer magneto-optic structures which provide improvements in recording sensitivity and readout reliability. These structures consist of combinations of magneto-optic, dielectric and metallic layers deposited in a closely-planar thin-film form. They absorb a significant fraction of the writing illumination, giving higher writing sensitivity. They also aim to increase the intensity of the (information-carrying) magneto-optic component of the scattered radiation, and decrease the intensity of the "ordinary" reflected radiation (i.e. that component polarization parallel to the incident radiation), giving higher readout reliability. However, further improvement is desired for many applications, especially in the application to computer memories. It is also desired to provide a structure that may be manufactured with higher yields and consequent lower manufacturing costs. The multilayer structures discussed above are difficult to manufacture to the required uniformity of properties within the total storage area of each structure, and between structures.